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I suspect the actual answer is 235.4 mpg or rather 1.2l/100km: they probably just took the official fuel consumption figure, multiplied it by 193.16 (100kms in 12000 miles) and multiplied by £1.32 as the price per litre. That gives you £305.96.

The BMW i3 is officially 0.6l/100km 0.6*1.32*193.16 gives you their £153 (actually £152.98).

So again those Green Car site figures are great for someone who follows the NEDC driving pattern, not so good for someone who keeps the hybrid car for journeys their electric car can't do. My driving pattern seems to fit the official figures fairly well for the Volt, but I suspect tha is as much chance as anything. It might well be that for a car with different electric range it would be a much worse fit.
 

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I was thinking about how this calculation reminded me of all those exams in school where you had to work out acceleration or current or something and you knew when you had the wrong answer because the correct answer always came out magically to some nice round number. In this case the 241 is an obviously wrong answer because it isn't a number we've seen before, whereas 235.4 is a fundamental constant that we all recognise and which significantly gives something very close to £1.32 for the petrol price.

Except that reminded me of another reason why 235.4 is completely meaningless. Never mind whether or not you drive to match the NEDC expectations, there's a hidden fudge factor in there. The problem is the official figures are calculated in l/100km and are mostly rounded to one decimal place. That's fine if you have a car doing 6.0 or 6.1l/100km, that's a 2% error, but the difference between 1.1 and 1.2l/100km is nearly 10% and by the time you're down to 0.5 or 0.6 you've got a possible 20% error.

Before I bought my Volt I went through the calculations from the Ampera blog and couldn't match the 235.4 until I realised you have to round the earlier result before doing the conversion to imperial. After converting to mpg they then compound things by introducing more apparently significant digits: in 235.4 the 3 is actually wrong never mind the 5.4.

Volvo look to have partly realised this is wrong as their V60 mpg figure 155.2mpg equates to 1.82l/100km, so they must have kept the second decimal place, but they still have spurious digits in the imperial figure.

Here is how you calculate the 235.4 for an Ampera:

Electric range (RE) = 83km
Consumption in mode A (CA) = 0
Consumption in mode B (CB) = 5.0 l/100km
Weight consumption (CW) = re/(re+25)*ca + 25/(re+25)*cb = 1.15(740) l/100km
Rounded to 1dp = 1.2 l/100km
Convert to imperial = 235.4 mpg

Without that all important rounding step the number would have been 244±3mpg (assuming CB is ±0.05). Given that in practice the range is usually in the range 48 to 81km (so 83km is really an upper limit) a more realistic official figure would have been 244mpg with error bars +3/-80mpg.
 

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Discussion Starter · #23 ·
Interesting :)

They say you can prove anything with numbers. They also say that there lies, damned lies and STATISTICS!

Nice post Duncan.
 

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In the real world for most people the numbers men nothing, but at lest I guess they are relative!

If you take into account the cost of economy 7 electric (which the cars calculated mpg doesn't) you get cost of about 250mpg when on battery only. If you use some petrol the cost is more than the calculated mpg which is what really matters to most motorists IMHO.

As I do an average of 60 miles a day during the week my mpg is going to sit around the 125 to 150 mpg mark I'm guessing (an electric post installed at work will help to get that up).

It's still a darn site better than the 40 mpg you'd get in an equivalent car!
 

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My lifetime is 59.9mpg at 22k miles :(
Julian ALONSO LEAD FOOT Thomas:D
How on earth did you get low? I can do a trip of about 180 miles and use less than 2.5 gallons (about 75mpg), I get over 55mpg when just on petrol.
 

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Discussion Starter · #29 ·
My lifetime is 73.7mpg now at 22k miles

Wish it were higher but I use the Ampera mainly for trips that I can't do or don't want to do in the Leaf so it isn't surprising as it is used mainly for long trips.

I am using it more for my daily runs to try to get the lifetime figure up. I'd like it to be over 100 by the time I sell it!
 

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I think the heavy foot is definitely a problem...!

I've treated it mainly like a petrol car, and until recently hadn't made much effort to maximise the EV bit.

It's done 4 or 5 round trips to Paris, several to the north of Scotland, several to Cornwall. And probably once per week is 200 miles round trip for work.

I'm feeling a bit sheepish seeing some of the other MPGs though!

image.jpg


This pic is one I took a couple of weeks ago when I first made an effort and made it into the 40 mile club!
 

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Just illustrates that the Government official test of fuel economy isn't a good measure of cars that can return such differing numbers depending on usage. But for now, it's all there is.
BMW isn't doing itself any favours by appearing to rely on that! I'd rather they gave their own set of figures if they thought customers were interested, and perhaps they don't.
In time, I hope the official tests will become better at describing hybrids' behaviour.
 
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